Organic photosensitive optoelectronic device with an exciton blocking layer

ABSTRACT

Organic photosensitive optoelectronic devices (“OPODs”) are disclosed which include an exciton blocking layer to enhance device efficiency. Single heterostructure, stacked and wave-guide type embodiments are disclosed. Photodetector OPODs having multilayer structures and an exciton blocking layer are also disclosed. Guidelines for selection of exciton blocking layers are provided.

This is a continuation-in-part application of U.S. patent applicationswith Ser. Nos. 09/136,342, 09/136,166, 09/136,377, 09/136,165,09/136,164 filed on Aug. 18, 1998.

FIELD OF INVENTION

The present invention generally relates to organic thin-filmphotosensitive optoelectronic devices. More specifically, it is directedto organic photosensitive optoelectronic devices, e.g., solar cells andvisible spectrum photodetectors, having an exciton blocking layer.

BACKGROUND OF THE INVENTION

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation. Photosensitive optoelectronic devices convert electromagneticradiation into electricity. Solar cells, also known as photovoltaic (PV)devices, are specifically used to generate electrical power. PV devicesare used to drive power consuming loads to provide, for example,lighting, heating, or to operate electronic equipment such as computersor remote monitoring or communications equipment. These power generationapplications also often involve the charging of batteries or otherenergy storage devices so that equipment operation may continue whendirect illumination from the sun or other ambient light sources is notavailable. As used herein the term “resistive load” refers to any powerconsuming or storing device, equipment or system.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g. crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein the term “semiconductor” denotes materialswhich can conduct electricity when charge carriers are induced bythermal or electromagnetic excitation. The term “photoconductive”generally relates to the process in which electromagnetic radiant energyis absorbed and thereby converted to excitation energy of electriccharge carriers so that the carriers can conduct, i.e., transport,electric charge in a material. The terms “photoconductor” and“photoconductive material” are used herein to refer to semiconductormaterials which are chosen for their property of absorbingelectromagnetic radiation of selected spectral energies to generateelectric charge carriers. Solar cells are characterized by theefficiency with which they can convert incident solar power to usefulelectric power. Devices utilizing crystalline or amorphous silicondominate commercial applications and some have achieved efficiencies of23% or greater. However, efficient crystalline-based devices, especiallyof large surface area, are difficult and expensive to produce due to theproblems inherent in producing large crystals without significantefficiency-degrading defects. On the other hand, high efficiencyamorphous silicon devices still suffer from problems with stability.Present commercially available amorphous silicon cells have stabilizedefficiencies between 4 and 8%. More recent efforts have focused on theuse of organic photovoltaic cells to achieve acceptable photovoltaicconversion efficiencies with economical production costs.

PV devices typically have the property that when they are connectedacross a load and are irradiated by light they produce a photogeneratedvoltage. When irradiated without any external electronic load, a PVdevice generates its maximum possible voltage, V open-circuit, orV_(OC). If a PV device is irradiated with its electrical contactsshorted, a maximum short-circuit current, or I_(SC), is produced. Whenactually used to generate power, a PV device is connected to a finiteresistive load and the power output is given by the current voltageproduct, I×V. The maximum total power generated by a PV device isinherently incapable of exceeding the product, I_(SC)×V_(OC). When theload value is optimized for maximum power extraction, the current andvoltage have values, I_(max) and V_(max) respectively. A figure of meritfor solar cells is the fill factor ff defined as: $\begin{matrix}{{ff} = \frac{I_{\max}V_{\max}}{I_{SC}V_{OC}}} & (1)\end{matrix}$where ff is always less than 1 since in actual use I_(SC) and V_(OC) arenever obtained simultaneously. Nonetheless, as ff approaches 1, thedevice is more efficient.

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This is represented symbolically asS₀+hv

S₀*. Here S₀ and S₀* denote ground and excited molecular states,respectively. This energy absorption is associated with the promotion ofan electron from a bound state in the valence band, which may be aπ-bond, to the conduction band, which may be a π*-bond, or equivalently,the promotion of a hole from the conduction band to the valence band.Inorganic thin-film photoconductors, the generated molecular state isgenerally believed to be an exciton, i.e., an electron-hole pair in abound state which is transported as a quasi-particle. The excitons canhave an appreciable life-time before geminate recombination, whichrefers to the process of the original electron and hole recombining witheach other as opposed to recombination with holes or electrons fromother pairs. To produce a photocurrent the electron-hole pair mustbecome separated. If the charges do not separate, they can recombine ina geminant recombination process, also known as quenching, eitherradiatively—re-emitting light of a lower than incident light energy-, ornon-radiatively—with the production of heat.

Either of these outcomes is undesirable in a photosensitiveoptoelectronic device. While exciton ionization, or dissociation, is notcompletely understood, it is generally believed to occur at defects,impurities, contacts, interfaces or other inhomogeneities. Frequently,the ionization occurs in the electric field induced around a crystaldefect, denoted, M. This reaction is denoted S₀*+M

e⁻+h⁺. If the ionization occurs at a random defect in a region ofmaterial without an overall electric field, the generated electron-holepair will likely recombine. To achieve a useful photocurrent, theelectron and hole must be collected separately at respective opposingelectrodes, which are frequently referred to as contacts. Excitondissociation occurs either in high electric field regions byfield-emission, or at an interface between, e.g., donor-like andacceptor-like materials such as copper phthalocyanine (CuPc) and3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), by chargetransfer. The latter can be viewed as an exothermic chemical reaction,i.e., a reaction in which some energy is released as vibrational energy.This reaction occurs because the energy separation of the dissociatedexciton, i.e., the energy difference between the free electron in, e.g.,PTCBI, and the free hole in, e.g., CuPc, is smaller that the energy ofthe exciton prior to dissociation.

Electric fields or inhomogeneities at a contact may cause an exciton toquench rather than dissociate, resulting in no net contribution to thecurrent. Therefore, it is desirable to keep photogenerated excitons awayfrom the contacts. This has the effect of limiting the diffusion ofexcitons to the region near the junction so that the junction associatedelectric field has an increased opportunity to separate charge carriersliberated by the dissociation of the excitons near the junction.

Here appreciation should be taken of some of the distinctions betweenorganic photosensitive optoelectronic devices (OPODs) and organic lightemitting devices (OLEDs). In an OLED, a bias is applied to a device toproduce a flow of holes and electrons into a device. In OLEDs, excitonsare generally formed which in time may either recombine radiatively ornonradiatively. In OLEDs, maximum radiative recombination is the desiredresult. In OPODs maximum exciton generation and dissociation is thedesired result. The differing objectives of the devices lead todiffering selection of materials and layer thicknesses. OPODphotosensitive materials are chosen for their absorption propertieswhile photoluminescent materials for OLEDs are chosen for their emissiveproperties.

To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a photovoltaicheterojunction. In traditional semiconductor theory, materials forforming PV heterojunctions have been denoted as generally being ofeither n, or donor, type orp, or acceptor, type. Here n-type denotesthat the majority carrier type is the electron. This could be viewed asthe material having many electrons in relatively free energy states. Thep-type denotes that the majority carrier type is the hole. Such materialhas many holes in relatively free energy states. The type of thebackground, i.e., not photogenerated, majority carrier concentrationdepends primarily on unintentional doping by defects or impurities. Thetype and concentration of impurities determine the value of the Fermienergy, or level, within the gap between the highest occupied molecularorbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),called the HOMO-LUMO gap. The Fermi energy characterizes the statisticaloccupation of molecular quantum energy states denoted by the value ofenergy for which the probability of occupation is equal to ½. A Fermienergy near the LUMO energy indicates that electrons are the predominantcarrier. A Fermi energy near the HOMO energy indicates that holes arethe predominant carrier. Accordingly, the Fermi energy is a primarycharacterizing property of traditional semiconductors and theprototypical PV heterojunction has traditionally been the p-n interface.

In addition to relative free-carrier concentrations, a significantproperty in organic semiconductors is carrier mobility. Mobilitymeasures the ease with which a charge carrier can move through aconducting material in response to an electric field. As opposed to freecarrier concentrations, carrier mobility is determined in large part byintrinsic properties of the organic material such as crystal symmetryand periodicity. Appropriate symmetry and periodicity can produce higherquantum wavefunction overlap of HOMO levels producing higher holemobility, or similarly, higher overlap of LUMO levels to produce higherelectron mobility. Moreover, the donor or acceptor nature of an organicsemiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA), may be at odds with the higher carrier mobility. For example,while chemistry arguments suggest a donor, or n-type, character forPTCDA, experiments indicate that hole mobilities exceed electronmobilities by several orders of magnitude so that the hole mobility is acritical factor. The result is that device configuration predictionsfrom donor/acceptor criteria may not be borne out by actual deviceperformance. Due to these unique electronic properties of organicmaterials, rather than designating them as “p-type” or “acceptor-type”and “n-type” or “donor-type”, the nomenclature of“hole-transporting-layer” (HTL) or “electron-transporting-layer” (ETL)is frequently used. In this designation scheme, an ETL willpreferentially be electron conducting and an HTL will preferentially behole transporting. The term “rectifying” denotes, inter alia, that aninterface has an asymmetric conduction characteristic, i.e., theinterface supports electronic charge transport preferably in onedirection. Rectification is associated normally with a built-in electricfield which occurs at the heterojunction between appropriately selectedmaterials.

The electrodes, or contacts, used in a photosensitive optoelectronicdevice are an important consideration. In a photosensitiveoptoelectronic device, it is desirable to allow the maximum amount ofambient electromagnetic radiation from the device exterior to beadmitted to the photoconductively active interior region. That is, it isdesirable to get the electromagnetic radiation to where it can beconverted to electricity by photoconductive absorption. This oftendictates that at least one of the electrical contacts should beminimally absorbing and minimally reflecting of the incidentelectromagnetic radiation. That is, such contact should be substantiallytransparent. When used herein, the terms “electrode” and “contact” referonly to layers that provide a medium for delivering photogenerated powerto an external circuit or providing a bias voltage to the device. Thatis, an electrode, or contact, provides the interface between thephotoconductively active regions of an organic photosensitiveoptoelectronic device and a wire, lead, trace or other means fortransporting the charge carriers to or from the external circuit. Theterm “charge transfer layer” is used herein to refer to layers similarto but different from electrodes in that a charge transfer layer onlydelivers charge carriers from one subsection of an optoelectronic deviceto the adjacent subsection. As used herein, a layer of material or asequence of several layers of different materials is said to be“transparent” when the layer or layers permit at least 50% of theambient electromagnetic radiation in relevant wavelengths to betransmitted through the layer or layers. Similarly, layers which permitsome but less that 50% transmission of ambient electromagnetic radiationin relevant wavelengths are said to be “semi-transparent”.

Electrodes or contacts are usually metals or “metal substitutes”. Hereinthe term “metal” is used to embrace both materials composed of anelementally pure metal, e.g., Mg, and also metal alloys which arematerials composed of two or more elementally pure metals, e.g., Mg andAg together, denoted Mg:Ag. Here, the term “metal substitute” refers toa material that is not a metal within the normal definition, but whichhas the metal-like properties that are desired in certain appropriateapplications. Commonly used metal substitutes for electrodes and chargetransfer layers would include doped wide bandgap semiconductors, forexample, transparent conducting oxides such as indium tin oxide (ITO),gallium indium tin oxide (GITO), and zinc indium tin oxide (ZITO). Inparticular, ITO is a highly doped degenerate n+ semiconductor with anoptical bandgap of approximately 3.2 eV rendering it transparent towavelengths greater than approximately 3900 Å. Another suitable metalsubstitute material is the transparent conductive polymer polyanaline(PANI) and its chemical relatives. Metal substitutes may be furtherselected from a wide range of non-metallic materials, wherein the term“non-metallic” is meant to embrace a wide range of materials providedthat the material is free of metal in its chemically uncombined form.When a metal is present in its chemically uncombined form, either aloneor in combination with one or more other metals as an alloy, the metalmay alternatively be referred to as being present in its metallic formor as being a “free metal”. Thus, the metal substitute electrodes of thepresent invention may sometimes be referred to as “metal-free” whereinthe term “metal-free” is expressly meant to embrace a material free ofmetal in its chemically uncombined form. Free metals typically have aform of metallic bonding that may be thought of as a type of chemicalbonding that results from a sea of valence electrons which are free tomove in an electronic conduction band throughout the metal lattice.While metal substitutes may contain metal constituents they are“non-metallic” on several bases. They are not pure free-metals nor arethey alloys of free-metals. When metals are present in their metallicform, the electronic conduction band tends to provide, among othermetallic properties, a high electrical conductivity as well as a highreflectivity for optical radiation.

A typical prior art photovoltaic device configuration is the organicbilayer cell. In the bilayer cell, charge separation predominantlyoccurs at the organic heterojunction. The built-in potential isdetermined by the HOMO-LUMO gap energy difference between the twomaterials contacting to form the heterojunction. The HOMO-LUMO energylevels for such a heterojunction are illustrated schematically in FIG. 1where 101 represents an anode, 102 represents an HTL layer, 103represents an ETL layer and 104 represents a cathode. The HOMO-LUMO gapoffset between the HTL and ETL produce an electric field around theHTL/ETL interface.

Herein, the term “cathode” is used in the following manner. In anon-stacked PV device or a single unit of a stacked PV device underambient irradiation and connected with a resistive load and with noexternally applied voltage, e.g., a solar cell, electrons move to thecathode from the adjacent photoconducting material. Similarly, the term“anode” is used herein such that in a solar cell under illumination,holes move to the anode from the adjacent photoconducting material,which is equivalent to electrons moving in the opposite manner. It willbe noted that as the terms are used herein anodes and cathodes may beelectrodes or charge transfer layers.

Organic PV devices typically have relatively low quantum yield (theratio of photons absorbed to carrier pairs generated, or electromagneticradiation to electricity conversion efficiency), being on the order of1% or less. This is, in part, thought to be due to the second ordernature of the intrinsic photoconductive process, that is, carriergeneration requires exciton generation, diffusion and ionization, asdescribed above. In order to increase these yields, materials and deviceconfigurations are desirable which can enhance the quantum yield and,therefore, the power conversion efficiency.

Thompson et al. in U.S. patent application Ser. No. 09/311,126 for “VeryHigh Efficiency Organic Light Emitting Devices Based onElectrophosphorescence” have described the use of an exciton blockinglayer to confine excitons to the emission layer in an organic lightemitting device (OLED) in order to increase the device efficiency. Inthe context of the present invention, an EBL is characterized by itsability to prevent the diffusion of excitons from an adjacent organiclayer into or across the EBL.

“Ultrathin Organic Films Grown by Organic Molecular Beam Deposition andRelated Techniques”, Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafterForrest, Chem. Rev. 1997) and Arbour, C, Armstrong, N. R., Brina, R.,Collins, G., Danziger, J.-P., Lee, P., Nebesny, K. W., Pankow, J.,Waite, S., “Surface Chemistries and Photoelectrochemistries of Thin FilmMolecular Semiconductor Materials”, Molecular Crystals and LiquidCrystals, 1990, 183, 307, (hereinafter Arbour et al.), disclose thatalternating thin multilayer stacks of similar type photoconductors couldbe used to enhance photogenerated carrier collection efficiency overthat using a single layer structure. Further, these sources describemultiple quantum well (MQW) structures in which quantum size effectsoccur when the layer thicknesses become comparable to the excitondimensions.

SUMMARY AND OBJECTS OF INVENTION

Several guidelines must be kept in mind in designing an efficientorganic photosensitive optoelectronic device. It is desirable for theexciton diffusion length, L_(D), to be greater than or comparable to thelayer thickness, L, since it is believed that most exciton dissociationwill occur at an interface. If L_(D) is less than L, then many excitonsmay recombine before dissociation. It is further desirable for the totalphotoconductive material thickness to be of the order of theelectromagnetic radiation absorption length, I/α (where α is theabsorption coefficient), so that nearly all of the radiation incident onthe solar cell is absorbed to produce excitons. However, the thicknessshould not be so large compared to the extent of the heterojunctionelectric fields that many excitons get generated in a field-free region.One reason for this is that the fields help to dissociate the excitons.Another reason is that if an exciton dissociates in a field-free region,it is more likely to suffer geminant recombination, or quenching, andcontribute nothing to the photocurrent. Further, electric fields mayexist at the electrode/semiconductor interfaces. These fields at theelectrode interfaces can also promote exciton quenching. Furthermore,the photoconductive layer thickness should be as thin as possible toavoid excess series resistance due to the high bulk resistivity oforganic semiconductors.

On the other hand, another countervailing consideration is that as theseparation between the exciton dissociating interface and the adjacentelectrodes increases, the electric field region around the interfacewill have a higher value over a greater volume. Since light absorptionincreases with increasing electric field strength, more excitons will begenerated. Also, the higher electric fields will also promote fasterexciton dissociation.

It has been suggested that one means for circumventing the diffusionlength limitation is to use thin cells with multiple or highly foldedinterfaces, such as can be achieved using nanotextured materials,polymer blends, closely spaced, repeated interfaces, or spatiallydistributed dissociation sites. To date, none of these proposals has ledto a significant improvement in overall performance of solar cells,particularly at high illumination intensities.

Accordingly, in the present invention higher internal and externalquantum efficiencies have been achieved by the inclusion in OPODs of oneor more exciton blocking layers (EBLs) to confine photogeneratedexcitons to the region near the dissociating interface and preventparasitic excilon quenching at a photosensitive organic/electrodeinterface. In addition to limiting the volume over which excitons maydiffuse, an EBL can also act as a diffusion barrier to substancesintroduced during deposition of the electrodes. In some circumstances,an EBL can be made thick enough to fill pinholes or shorting defectswhich could otherwise render an OPOD non-functional. An exciton blockinglayer can therefore help protect fragile organic layers from damageproduced when electrodes are deposited onto the organic materials.

It is believed that the EBLs comprising the present invention derivetheir exciton blocking property from having a LUMO-HOMO energy gapsubstantially larger than that of the adjacent organic semiconductorfrom which excitons are being blocked. The thus confined excitons areprohibited from existing in the EBL due to quantum energy considerationsWhile it is desirable for the EBL to block excitons, it is not desirablefor the EBL to block all charge carrying quanta as well. However, due tothe nature of the adjacent energy levels an EBL will necessarily blockone sign of charge carrier. By design, an EBL will always exist betweentwo adjacent layers, usually an organic photosensitive semiconductorlayer and a electrode or charge transfer layer. The adjacent electrodeor charge transfer layer will be in context either a cathode or ananode. Therefore, the material for an EBL in a given position in adevice will be chosen so that the desired sign of carrier will not beimpeded in its transport to the electrode or charge transfer layer.Proper energy level alignment ensures that no barrier to chargetransport exists, preventing an increase in series resistance. It shouldbe appreciated that the exciton blocking nature of a material is not anintrinsic property. Whether a given material will act as an excitonblocker depends upon the relative HOMO and LULMO levels of the adjacentorganic photosensitive material. Therefore, it is not possible toidentify a class of compounds in isolation as exciton blockers withoutregard to the device context in which they may be used. However, withthe teachings herein one of ordinary skill in the art may identifywhether a given material will function as an exciton blocking layer whenused with a selected set of materials to construct an OPOD.

For example, FIGS. 2A through 2C illustrate three types of bilayer OPODshaving one or more EBLs to suppress undesired exciton diffusion andenhance device efficiency. These figures schematically depict therelative energy levels of the various materials comprising variousembodiments of an OPOD cell having one or more EBLs. The lines in eachfigure at the ends represent the work function of the electrodes orcharge transfer layers at the ends. The shaded boxes represent therelative LUMO-HOMO energy gaps of the various constituent layers of theOPOD.

With regard to FIG. 2A, OPOD device 2A00 comprises an anode layer 2A01,such as indium tin oxide (ITO), a hole transporting layer (HTL) 2A02,such as CuPc which is believed to have a LUMO-HOMO separation ofapproximately 1.7 eV, an electron transporting layer (ETL) 2A03, such asPTCBI which is also believed to have a LUMO-HOMO separation ofapproximately 1.7 eV, an exciton blocking layer (EBL) 2A04, such as2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproineor BCP) which is believed to have a LUMO-HOMO separation of about 3.5eV, and a cathode layer 2A05, such as silver. It should be appreciatedthat the larger LUMO-HOMO energy gap in EBL 2A04 will prohibit diffusionof excitons from ETL 2A03 into EBL 2A04. Also, coincidentally EBL 2A04would block the transit of holes from ETL 2A03 toward the cathode due tothe unfavorable gap between the HOMO levels of ETL 2A03 and EBL 2A04,i.e., the higher ionization potential of the EBL. However, this effectis thought to be of little consequence since the internal electric fieldgenerated around the HTUJETL interface will tend to drive holes towardsanode layer 1 so that there are relatively few holes near the ETLUEBLinterfacial region. One result of this hole blocking aspect is that EBL2A04 is optimally a cathode side EBL. Note also that there isincidentally a slightly unfavorable LUMO gap for electrons at theETU/EBL interface in the illustrated example using PTCBI as the ETL andBCP as the EBL. Optimally, it is desirable for a material used as acathode side EBL to have a LUMO level closely matching the LUMO level ofthe adjacent ETL material so that the undesired barrier to electrons isminimized.

With regard to FIG. 2B, the analogous situation of an anode side EBL isdepicted. OPOD device 2B00 comprises an anode layer 2B101, such asindium tin oxide (ITO), an exciton blocking layer (EBL) 2B02, such as4,4′,4″-tris{N,-(3-methylphenyl)-N-phenylamino}triphenylamine (m-MTDATA)or polyethylene dioxythiophene (PEDOT). The LUMO-HOMO separations form-MTDATA and PEDOT are not precisely known but believed to be such asdepicted in FIG. 2B. The OPOD further comprises a hole transportinglayer (HTL) 2B03, such as CuPc, an electron transporting layer (ETL)2B04, such as PTCBI, and a cathode layer 2B05, such as silver. It shouldbe appreciated that the larger LUMO-HOMO energy gap in EBL 2B02 willprohibit diffusion of excitons from HTL 2B03 into EBL 2B02. Also,coincidentally EBL 2B02 would block the transit of electrons from HTL2B03 toward the cathode due to the unfavorable gap between the LUMOlevels of HTL 2B03 and EBL 2B02, i.e., the higher LUMO level of the EBL.However, this effect is thought to be of little consequence since theinternal electric field generated around the HTUJETL interface will tendto drive electrons towards cathode layer 2B05 so that there arerelatively few electrons near the HTUIEBL interfacial region. One resultof this electron blocking aspect is that EBL 2B02 is optimally an anodeside EBL.

Finally, in FIG. 2C, the various relative energy layers of an OPOD 2C00having both anode side and cathode side EBLs is illustrated. An anodelayer 2C01, such as ITO, an anode side EBL 2C02, such as m-MTDATA orPEDOT, a HTL 2C03, such as CuPc, an ETL 2C04, such as PTCBI, a cathodeside EBL 2C05, such as BCP, and a cathode layer 2C06, such as silver.Accordingly, with both an anode side EBL and a cathode side EBL,excitons generated within HTL 2C03 and ETL 2C04 are effectively confineduntil they preferably dissociate or undesirably quench.

A multilayer structure like that whose energy state structure isdepicted in FIG. 2D is a highly efficient photodetector. In FIG. 2D,2D01 is a transparent anode, e.g., ITO, which is adjacent to one ofseveral HTL, e.g., CuPc, layers 2D02. Between the HTL layers 2D02 andadjacent to exciton blocking layer 2D04 are several ETL, e.g., PTCBI,layers 2D03. Exciton blocking layer 2D04 is BCP in this example. Excitonblocking layer 2D04 is adjacent to cathode 2D05 which is, e.g., silver.Arbour et al and Forrest, Chem. Rev. 1997 suggested that the numerousHTL-ETL interfaces can provide efficient free carrier generation when abias is provided to extract the carriers from the device. Arbour andForrest did not, however, suggest the use of an exciton blocking layeras described herein to further enhance the efficiency in such devices.

OPODs operating without a bias and including an EBL in accordance withthe present invention can be made very thin without severe loss ofphotocurrent. Accordingly, OPODs including EBLs may be used incombination with the highly efficient OPODs of the U.S. patentapplications of Forrest et al. with Ser. Nos. 09/136,342, 09/136,166,09/136,377, 09/136,165, 09/136,164 (hereinafter collectively “ForrestOPOD Appls.”) which are incorporated herein by reference in theirentirety. Stacked OPODs including EBLs and having numerous subcellsand/or including a waveguide configuration may be constructed in accordwith the present invention to achieve high internal and external quantumefficiencies.

When the term “subcell” is used hereafter, it refers to an organicphotosensitive optoelectronic construction which may include an excitonblocking layer in accordance with the present invention. When a subcellis used individually as a photosensitive optoelectronic device, ittypically includes a complete set of electrodes, i.e., positive andnegative. As disclosed herein, in some stacked configurations it ispossible for adjacent subcells to utilize common, i.e., shared,electrode or charge transfer layers. In other cases, adjacent subcellsdo not share common electrodes or charge transfer layers. The term“subcell” is disclosed herein to encompass the subunit constructionregardless of whether each subunit has its own distinct electrodes orshares electrodes or charge transfer layers with adjacent subunits.Herein the terms “cell”, “subcell”, “unit”, “subunit”, “section”, and“subsection” are used interchangeably to refer a photoconductive layeror set of layers and the adjoining electrodes or charge transfer layers.As used herein, the terms “stack”, “stacked”, “multisection” and“multicell” refer to any optoelectronic device with multiple layers of aphotoconductive material separated by one or more electrode or chargetransfer layers.

Since the stacked subcells of the solar cell may be fabricated usingvacuum deposition techniques that allow external electrical connectionsto be made to the electrodes separating the subcells, each of thesubcells in the device may be electrically connected either in parallelor in series, depending on whether the power and/or voltage generated bythe solar cell is to be maximized. The improved external quantumefficiency that may be achieved for stacked solar cell embodiments ofthe present invention may also be attributed to the fact that thesubcells of the stacked solar cell may be electrically connected inparallel since a parallel electrical configuration permits substantiallyhigher fill factors to be realized than when the subcells are connectedin series.

Although the high series resistance of photoconductive organic materialsinhibits use of subcells in a series configuration for high powerapplications, there are certain applications, for example, in operatingliquid crystal displays (LCD), for which a higher voltage may berequired, but only at low current and, thus, at low power levels Forthis type of application, stacked, series-connected solar cells may besuitable for providing the required voltage to the LCD. In the case whenthe solar cell is comprised of subcells electrically connected in seriesso as to produce such a higher voltage device, the stacked solar cellmay be fabricated so as to have each subcell producing approximately thesame current so to reduce inefficiency. For example, if the incidentradiation passes through in only one direction, the stacked subcells mayhave an increasing thickness with the outermost subcell, which is mostdirectly exposed to the incident radiation, being the thinnest.Alternatively, if the subcells are superposed on a reflective surface,the thicknesses of the individual subcells may be adjusted to accountfor the total combined radiation admitted to each subcell from theoriginal and reflected directions.

Further, it may be desirable to have a direct current power supplycapable of producing a number of different voltages. For thisapplication, external connections to intervening electrodes could havegreat utility. Accordingly, in addition to being capable of providingthe maximum voltage that is generated across the entire set of subcells,an exemplary embodiment the stacked solar cells of the present inventionmay also be used to provide multiple voltages from a single power sourceby tapping a selected voltage from a selected subset of subcells.

Representative embodiments may also comprise transparent charge transferlayers. As described herein charge transfer layers are distinguishedfrom ETL and HTL layers by the fact that charge transfer layers arefrequently, but not necessarily, inorganic and they are generally chosennot to be photoconductively active.

Embodiments of the present invention may include, as one or more of thetransparent electrodes of the optoelectronic device, a highlytransparent, non-metallic, low resistance cathode such as disclosed inU.S. patent application Ser. No. 09/054,707 to Parthasarathy et al.(“Parasarathy '707”), or a highly efficient, low resistancemetallic/non-metallic composite cathode such as disclosed in U.S. Pat.No. 5,703,436 to Forrest et al. (“Forrest '436”). Each type of cathodeis preferably prepared in a fabrication process that includes the stepof sputter depositing an ITO layer onto either an organic material, suchas copper phthalocyanine (CuPc), PTCDA and PTCBI, to form a highlytransparent, non-metallic, low resistance cathode or onto a thin Mg:Aglayer to form a highly efficient, low resistance metallic/non-metalliccomposite cathode. Parasarathy '707 discloses that an ITO layer ontowhich an organic layer had been deposited,.instead of an organic layeronto which the ITO layer had been deposited, does not function as anefficient cathode.

It is an object of the present invention to provide an OPOD and an OPODsubcell comprising one or more exciton blocking layers to increase theinternal quantum efficiency of the OPOD or OPOD subcell.

It is an object of the present invention to provide an OPOD capable ofoperating with a high external quantum efficiency and comprising stackedOPOD subcells

It is another object of the present invention to provide a stacked OPODcapable of operating with an external quantum efficiency that approachesthe maximum internal quantum efficiency of an optimal OPOD subcell.

Another object of the present invention is to provide an OPOD withimproved absorption of incident radiation for more efficientphotogeneration of charge carriers.

It is a further objective of the present invention to provide an OPODwith an improved V_(oc) and an improved I_(sc).

Another object of the present invention is to provide a stacked OPODhaving parallel electrical interconnection of the subcells.

A further object of the present invention is to provide a stacked OPODcomprised of multiple organic OPOD subcells with transparent electrodesand having a substantially reflective bottom layer to increase overallelectromagnetic radiation absorption by capturing the electromagneticradiation reflected by the bottom layer.

A further object of the present invention is to provide a waveguideconfiguration OPOD having an exciton blocking layer.

Yet another object of the present invention is to provide OPODsincluding a conductive or an insulating substrate.

A further object of the present invention is to provide OPODs includinga rigid or a flexible substrate.

A further object of the present invention is to provide OPODs whereinthe organic materials used are polymeric or non-polymeric thin films.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description of exemplaryembodiments taken in conjunction with the attached drawings.

FIG. 1 illustrates the relative energy levels in a typical prior artdevice.

FIGS. 2A-2C illustrate the relative energy levels in exemplaryembodiments of the invention having a cathode side EBL, an anode sideEBL, or both.

FIG. 2D illustrates the relative energy levels in an exemplaryembodiment of a multilayer photodetector having an EBL on the cathodeside.

FIG. 3 depicts an exemplary OPOD in accord with the present invention.

FIG. 4 is a graph of calculated exciton density profiles comparing theeffect of an EBL with the effect of an exciton quenching interface.

FIG. 5 depicts an effect of the EBL to shift the active region ofexciton dissociation to the region of maximum optical electric fieldintensity.

FIG. 6 is a graph of measured external quantum efficiency (η_(ext)) atλ=620 nm of ITO/CuPc/PTCBI/BCP/Ag devices as a function of differentlayer thicknesses.

FIG. 7 is a graph of current vs. voltage (I-V) measurements of a thindevice incorporating an EBL (ITO/150 Å CuPc/60 Å PTCBI/150 ÅBCP:PTCBI/800 Å Ag) under different intensities of AM1.5 spectralillumination.

FIG. 8 illustrates an exemplary embodiment of a stacked OPOD in accordwith the present invention.

FIG. 9 illustrates an exemplary embodiment of a wave-guide geometry typeOPOD in accord with the present invention.

FIG. 10 schematically depicts a stacked OPOD having an exciton blockinglayer.

FIG. 11 depicts a waveguide type OPOD having an exciton blocking layer.

FIG. 12 is a top down view of FIG. 12 through line A-A.

FIG. 13 is a graph of current/voltage characteristics of some multilayerphotodetectors in accord with the present invention.

FIG. 14 is a plot of some efficiency and absorption data as a functionof incident wavelength for a multilayer photodetector in accord with thepresent invention.

FIG. 15 is plot of external quantum efficiency as a function of voltagefor some multilayer photodetectors in accord with the present invention.

DETAILED DESCRIPTION

Devices have been constructed and example data recorded for exemplaryembodiments of the present invention, in particular, the device depictedin FIG. 3.

In FIG. 3, OPOD 300 has cathode 301 of a suitable electrode materialsuch as silver, an EBL 302 of a suitable material such as BCP, an ETL303 such as PTCBI, a HTL 304 such as CuPc, and an anode of a suitableelectrode material such as ITO. In such a device at least one of theelectrodes must be transparent to allow the admission of electromagneticradiation. Hereafter, calculations and data are presented from actualdevices and compared to prior art and theory. Other prospectiveembodiments of devices in accord with the present invention are alsodescribed.

Exemplary embodiments were fabricated on pre-cleaned glass substratescoated with a ˜1500 Å thick transparent, conducting indium-tin-oxide(ITO) anode (with a sheet resistance of 40 Ω/sq.). Prior to deposition,the organic materials were purified in three cycles using thermalgradient sublimation. Films were grown onto the ITO employing ultrahighvacuum (1×10⁻¹⁰ Torr) organic molecular beam deposition in the followingsequence: 30 Å to 600 Å thick films of donor-like copper-phthalocyanine(CuPc) was followed by a 30 Å to 600 Å thick films of acceptor-like3,4,9,10-perylenetetracarboxylic bisimidazole (PTCBI). Next, a 100 Å to200 Å thick film of the bathocuproine (BCP) EBL was deposited. Here, BCPwith a 3.5 eV energy gap, has previously been shown to be an effectiveexciton blocker which can easily transport electrons to the top 800 Åthick Ag cathode (evaporated at 1×10⁻⁶ Torr, through a shadow mask with1 mm diameter openings, in a separate vacuum chamber after exposing theorganics to the atmosphere) from the adjoining PTCBI layer. Allelectrical measurements of the completed OPODs were performed in airunless otherwise specified.

FIG. 4 is a theoretical calculation of exciton density as a function ofposition in a photosensitive organic material under two differentboundary conditions for the right interface. Both exciton profiles arefor a 60 Å thick layer of an organic photosensitive material, e.g.,PTCBI, assuming uniform generation of excitons throughout the film. Theuniform generation follows from assuming L_(D)<<α⁻¹, i.e., theabsorption length is much greater than the exciton diffusion length.Here, the exciton diffusion length, L_(D) was taken to be 30 Å. The fullline assumes an EBL to the right hand side. The dashed line has aquenching interface at the right hand side. In both cases, the left handinterface is the intentional exciton sink (for example the CuPc/PTCBIinterface in embodiment 300). In a device in accordance with the presentinvention such as 300, excitons are purposefully lost at the excitonsink interface where they are converted to pairs of free chargecarriers. The much higher value of the solid curve at the right end ofthe graph illustrates that the exciton recombination rate at theinterface with the EBL is much lower and is preferably negligible.

FIG. 5 illustrates another beneficial attribute of an EBL in certainOPOD configurations. The active region is predicted to shift away fromthe region of vanishing optical electric field when an exciton blockinglayer is inserted in an OPOD having a metallic back electrode, e.g., Ag.As can be seen from this graph, the insertion of an exciton blockinglayer, e.g., BCP, effectively increases the average value of the squareof the electric component of the optical field in the active regions ofthe device. The optical electric field profile depicted is conceptualand arises because of boundary conditions at the metallic interfacewhich correspond to optical reflection. Note that the actual opticalelectric field profile will depend on the dielectric constant of therespective layers traversed by the incident light and varies for thedifferent wavelengths of the incident radiation. While the details mayvary, it is apparent that inserting an EBL layer in an exemplary deviceconfiguration, such as depicted in FIG. 5, provides some additionalseparation between the back reflecting electrode and the heterojunction.This is likely to put the heterojunction in a region of higher opticalelectric field. The shift of the optical electric field increases theabsorption of the active layers, and hence the photon collectionefficiency. This does not affect the internal quantum efficiency.However, in a device wherein the captured light is reflected multipletimes through the photoactive layers, such as the waveguideconfiguration described later herein, it does affect the required numberof passes the light must make to obtain high external efficiencies. Instacked devices which generally lack reflective layers, this absorptionenhancement effect will not be present since the mean square value ofthe optical electric field will generally be a purely decaying functionof the penetration depth into the device of the incident radiation.

FIG. 6 shows the external quantum efficiency (η_(ext)) at λ=620 nm(corresponding to an absorption peak of CuPc) of several exemplarydevices embodying the present invention, e.g., ITO/CuPc/PTCBI/BCP/Ag, asa function of different layer thicknesses. For devices with 300 521PTCBI and 100 Å BCP (filled circles), an increase in η_(EXT)is observedas CuPc layer thicknesses is reduced. Similarly, for devices with 300 ÅCuPc and 100 Å BCP (filled squares), an increase in η_(EXT) is observedat λ=540 nm (an absorption peak of PTCBI) as the PTCBI layer thicknessesis reduced. If the BCP-EBL is omitted, the PV cell photocurrent responseis significantly reduced for the thinnest cells, as shown for deviceswith 300 Å CuPc and a PTCBI layer of various thicknesses (open squares).Note that this BCP layer allows for the fabrication of devices withtotal active layer thicknesses of only 60 Å without electrical shorts.In addition, electrical measurements show that the series resistance ofthe cells remains unaffected for BCP layers as thick as 200 Å BCP. Themonotonic increase of η_(EXT) and the even greater increase of η_(INT)with decreasing layer photoactive layer thicknesses in the presence ofthe EBL provide striking evidence that excitons must diffuse to theheterointerface for efficient dissociation and subsequent chargecollection. The decreasing external quantum efficiency for thicker filmsis then solely due to the increased absorption in inactive regions (i.e.regions further than one diffusion length from the heterointerface). Itis thought that in addition to keeping photogenerated excitons away fromthe quenching Ag interface, the EBL also helps prevent the incurrence ofAg clusters into the electron transport layer. Such clusters can causeshorting defects and provide additional quenching sites for excitons.

The current vs. voltage (I-V) measurements of another device in accordwith the present invention, e.g., ITO/150 Å CuPc/60 Å PTCBI/150 ÅBCP:PTCBI/800 Å Ag, under different intensities of AM1.5 spectralillumination are shown in FIG. 7. Simulated spectral illumination ofvariable intensity was obtained under a nitrogen ambient using a 150 WXe arc lamp equipped with AM1.5 and neutral density filters to attenuatethe beam. (The optical power was measured using a calibrated Siphotodiode from Newport, Inc. The Xe arc lamp was from Oriel.) The I-Vresponse is characterized by a shunt resistance (R_(o)A, where A isthejunction area) of 20±2 kΩ-cm², and a small series resistance of 30±10Ω-cm². The dark current follows the expression for a classical p-njunction diode with an ideality factor of n=1.4-1.7. These valuescompare favorably with amorphous silicon cells and are a significantimprovement over previous reports of polymer thin film cells.

It should be appreciated that the BCP layer was doped with ˜10% (byweight) of PTCBI. It is thought that the BCP as deposited in the presentdevices is amorphous. It is thought that good quality crystalline wouldalso function as an EBL and might have better electron transportproperties. However, it may be difficult or inefficient to prepare goodcrystalline material. The present apparently amorphous BCP excitonblocking layers do exhibit film recrystallization, which is especiallyrapid under high light intensities. The resulting morphology change topolycrystalline material results in a lower quality film with possibledefects such as shorts, voids or intrusion of electrode material.Accordingly, it has been found that doping of some EBL materials, suchas BCP, that exhibit this effect with a suitable, relatively large andstable molecule can stabilize the EBL structure to prevent performancedegrading morphology changes. It should be further appreciated thatdoping of an EBL which is transporting electrons in a giving device witha material having a LUMO energy level close to that of the EBL will helpinsure that electron traps are not formed which might produce spacecharge build-up and reduce performance. Additionally, it should beappreciated that relatively low doping densities should minimize excitongeneration at isolated dopant sites. Since such excitons are effectivelyprohibited from diffusing by the surrounding EBL material, suchabsorptions reduce device photoconversion efficiency.

The dependence of the performance parameters for an OPOD in accord withthe present invention on the AM 1.5 optical flux is shown in FIG. 8. Theshort-circuit current (I_(SC)) is linear with illumination intensity,indicating that even at the highest illumination levels of ˜15 suns, nosignificant space charge build-up occurs. The open circuit voltage(V_(OC)) increases monotonically until it reaches a plateau ofV_(OC)=0.54 V for illumination intensities >10 suns. The fill factor(ff), as defined in equation 1 and illustrated in FIG. 7, approaches0.57 at low intensities, a value typical for conventional inorganicsolar cells, and exceeds the typical value of ff<0.35 found inotherorganic PVs even at the highest illumination intensitiesconsidered. Since ff decreases with increasing V_(OC) and lightintensity, the external power conversion efficiency (η_(P)) at AM1.5 isonly a slowly varying function ofthe illumination intensity, reaching amaximum of η_(P)=(1.1±0.1)% over a broad plateau extending from 0.1 to10 suns. These results represent a significant improvement over previousdemonstrations of thin film organic PV cells, and for the first time,efficient operation under simulated solar illumination of multiple sunsis achieved without a decrease in power conversion efficiency.

FIG. 9 shows the photocurrent action spectrum (η_(EXT), solid circles)at zero bias (short circuit condition) of a device in accord with thepresent invention. The device structure was ITO/90 Å CuPc/90 Å PTCBI/100Å BCP/Ag (a non-doped EBL). The excellent match of the action spectrumto the solar spectrum is apparent. The action spectrum also is welldescribed by the sum of the absorption spectra of the organic films(weighted by the optical flux incident on each absorbing layer via theglass substrate), corroborating the assumption that the excitonicspecies is the intermediate state between photon absorption and theseparated electron-hole pair. Now, η_(EXT) is observed to increaselinearly with reverse bias, with the slope of the photocurrent versusapplied voltage dependent only on the PTCBI layer thickness. Further,the increase in η_(EXT) follows the PTCBI absorption spectrum.Accordingly, the dependence of photocurrent on voltage is thought to bedue to intrinsic photoconduction in PTCBI, i.e., exciton dissociation inthe film bulk.

FIG. 9 also plots the spectral dependence of the calculated internalquantum efficiency (η_(INT), open circles), with a maximum efficiency of25% observed for PTCBI and CuPc thicknesses of 90 Å. It should beappreciated that an internal quantum efficiency of ˜25% is consistentwith analytical solutions to the exciton diffusion equation for thegeometry of interest. This is a strong indication that the photonharvesting efficiency is limited only by exciton diffusion.

Due to the thin photoactive layers of the embodiments described so far,device geometries which provide a means for increasing the effectivethickness of the absorbant layers are preferable. One such configurationis a stacked OPOD. A stacked OPOD 1000 comprising exciton blockinglayers is schematically illustrated in FIG. 10. Layers 1001, 1003, 1005,and 1007 are electrodes or charge transfer layers which may be metal ormetal substitutes as described above and in the Forrest OPOD Appls.Sections 1002, 1004 and 1006 represent photosensitive heterostructuressuch as those depicted in FIGS. 2A-2C which form OPOD subcells of thestacked OPOD 1000. Electrode or charge transfer layers 1003 and 1005 arepreferably transparent while at least one of layers 1001 or 1007 ispreferably transparent so that light incident upon either the uppermostor lowermost face of device 1000 will be admitted into the device forphotoconversion. Layer 1008 is a conventional substrate material such asglass, metals, plastics, etc. The substrate is transparent when light isto be admitted through the substrate. Optionally, one of 1001 or 1007,may be reflective or an additional reflective layer may be added on theface opposite the incident light. Additional subcells may beincorporated in such a stacked structure. As described in the ForrestOPOD Appls., the subcells of device 1000 may be electrically connectedin series or parallel or in combinations of series and parallel. Also,an exciton blocking layer may be incorporated into other heterostructuredesigns such as the unilayer and multilayer structures described in theForrest OPOD Appls.

Alternatively, it is apparent from measurements of η_(INT) that anincreased η_(P) can be achieved in a concentrator configuration wherephotons are forced to make multiple passes through the thin absorbingregion. It should be appreciated regarding embodiment 1000 that lightincident on a transparent face of the device can generally be reflectedonce off of an opposite interior reflecting layer and then eitherabsorbed or possibly transmitted back out of the device. Deviceconfigurations are described in co-pending U.S. patent applicationAttorney docket number 10644/50501 (“50501 Application”) (incorporatedherein by reference) which cause any light admitted to a device to bereflected multiple times to increase absorption efficiency.

A device in accord with the present invention (depicted in FIG. 11)having a reflective Ag layer 1101 with a small aperture on the substratesurface was used to demonstrate this increase in efficiency. Transparentlayer 1102, of, for example, glass or plastic, was much wider than theoptical coherence length. Transparent anode of degenerately doped ITO1103 permitted the light to reach electronically active layers 1104.Metallic cathode 1105 reflected unabsorbed light. Concentrated radiation(10 suns at AM1.5) was focused on an aperture in reflective layer 1101and formed a near normal incidence beam which reflected several timesbetween the cathode and Ag reflecting surface 1101, with each passsuffering additional absorption by a CuPc/PTCBI bilayer adjacent to aBCP EBL (shown collectively as 1104 and like FIG. 2A). FIG. 12 uses thesame reference numerals as FIG. 11 to illustrate the circular aperturein reflective layer 1101 since layer 1102 can be seen through theaperture in this view of embodiment 1100 taken along line A-A in FIG.11. Using this technique, an external power efficiency of η_(P)=2.4±0.3%was measured for a cell with 60 Å CuPc, 60 Å PTCBI and 150 Å BCP. Thisis believed to be the highest external power conversion efficiency atAM1.5 reported for an organic thin film photovoltaic cell. Note alsothat due to the small top electrode, not all of the incident radiationwas trapped in this example. Hence, the power efficiency obtainedrepresents a lower limit. By placing multiple, parallel connected cells(such as those disclosed in the Forrest OPOD Appls.) in a reflectingbeam path, it is believed that, given a sufficient number of passes,efficiencies exceeding 3% can be achieved under improved light trapping.It should be appreciated that this device structure is particularly ableto take advantage of the optical electric field enhancement depicted inFIG. 5.

It should also be appreciated that better control of the growth processwould allow one to grow thinner, and thus more efficient devices.Further optimization of the transparency and reflectivity of theelectrodes will reduce parasitic absorption. In addition, tuning theenergy level alignment of the electron-donor and acceptor materials suchthat the binding energy of the exciton (˜1 eV) more closely matches theopen-circuit voltage will further enhance device performance. It isbelieved that ˜80% internal efficiencies, corresponding to ˜8% powerconversion efficiencies are within the reach of such optimized organicsolar cells.

It should be appreciated that the advantages of an OPOD having an EBL ina waveguide type device were demonstrated using simulated concentratedsun light. Nonetheless, actual sun light can be concentrated anddirected into the photoactive regions of an OPOD as described in the50501 Application.

FIGS. 13-15 illustrate data from examples of a multilayer photodetectorhaving an EBL such as that of FIG. 2D. The HTL layer 2D02 adjacent anelectrode and the ETL layer 2D03 adjacent an electrode are typicallythicker than the multiple pairs of HTUETL layers in the device interioraway from electrodes. Typically then, layer 2D02 adjacent cathode 2D05is about 30-100 Å of CuPc. Similarly, layer 2D03 adjacent anode 2D01 istypically 30-100 Å of PTCBI. EBL 2D04 is, for example, 50-200 Å of BCP.The multiple pairs of HTL/ETL layers can have ETL and HTL layers having,e.g., 2-10 Å thickness, with the pairs repeated from 2 to 50 times. FIG.13 shows current-voltage for a multilayer photodetector and shows inthis example that 20 HTL/ETL pairs produces a higher current responsethat 40 such interfaces. FIG. 14 shows quantum efficiency and absorptiondata for such multilayer photodetectors and illustrates a broad flatspectral response. FIG. 15 shows external quantum efficiency data forphotodetectors having 20 or 40 HTL/ETL pairs and that the 20 layerdevice has a higher external quantum efficiency. In the 20 and 40 pairphotodetectors, the overall device thickness was not increased by thesame factor as the number of pairs, i.e., 2 times, so the photosensitivelayers forming the pairs were much thinner for the 40 pair device. It isbelieved that the current response and quantum efficiency were betterfor the 20 pair device, because the thinness of the HTL and ETL layersin the 40 pair device may have caused the layers to begin to lose theircharacter as discrete layers. Instead it is believed that the materialsformning the layers may have somewhat intermixed producing the somewhatpoorer performance.

Thus, there has been described and illustrated herein an organicphotosensitive optoelectronic device and method for producing the same.Those skilled in the art, however, will recognize that manymodifications and variations besides those specifically mentioned may bemade in the apparatus and techniques described herein without departingsubstantially from the concept of the present invention. Accordingly, itshould be clearly understood that the form of the present invention asdescribed herein is exemplary only and is not intended as a limitationon the scope of the present invention.

1. An organic photosensitive optoelectronic device comprising: twoelectrodes in superposed relation; a hole transport layer between thetwo electrodes, the hole transport layer formed of a firstphotoconductive organic semiconductor material; an electron transportlayer between the two electrodes and adjacent to the hole transportlayer, the electron transport layer formed of a second photoconductiveorganic semiconductor material; and at least one exciton blocking layerbetween the two electrodes and adjacent to at least one of theelectrodes.
 2. The device of claim 1 wherein one exciton blocking layeris between the electron transport layer and the electrode adjacent theexciton blocking layer.
 3. The device of claim 1 wherein one excitonblocking layer is between the hole transport layer and the electrodeadjacent the exciton blocking layer.
 4. The device of claim 1 whereinthe at least one exciton blocking layer is a first exciton blockinglayer and a second exciton blocking layer, the first exciton blockinglayer being between the electron transport layer and the electrodeadjacent the first exciton blocking layer, the second exciton blockinglayer being between the hole transport layer and the electrode adjacentthe second exciton blocking layer.
 5. The device of claim 1 wherein thefirst photoconductive organic semiconductor material and the secondphotoconductive organic semiconductor material are selected to havesubstantial spectral sensitivity in the visible spectrum.
 6. The deviceof claim 2 wherein: the electron transport layer is PTCBI; the holetransport layer is CuPc; and the exciton blocking layer is BCP.
 7. Thedevice of claim 3 wherein: the electron transport layer is PTCBI; thehole transport layer is CuPc; and the exciton blocking layer is one ofthe group consisting of m-MTDATA or PEDOT.
 8. The device of claim 1wherein the electron transport layer, the hole transport layer, and theexciton blocking layer are disposed between two parallel planarreflective surfaces which form a waveguide.
 9. The device of claim 8wherein one of the two reflective surfaces has an aperture to admitlight incident upon the device.
 10. The device of claim 8 having atransparent opening between the two reflective surfaces so that light isadmitted to the device from a direction parallel to the planes of thereflective surfaces.
 11. A stacked organic photosensitive optoelectronicdevice comprised of a plurality of photosensitive optoelectronicsubcells wherein at least one subcell includes an exciton blockinglayer.
 12. The device of claim 10 wherein the subcell including anexciton blocking layer has an electron transport layer including PTCBIand a hole transport layer adjacent the electron transport layer andincluding CuPc.
 13. The device of claim 11 wherein the subcell includingan exciton blocking layer, the exciton blocking layer includes BCP andis adjacent the electron transport layer and in spaced opposition to thehole transport layer.
 14. The device of claim 11 where, in the subcellincluding an exciton blocking layer, the exciton blocking layer isselected from the group consisting of m-MTDATA or PEDOT, and is adjacentthe hole transport layer and in spaced opposition to the electrontransport layer.
 15. An organic photodetector comprising: a cathode andan anode in superposed relation; a plurality of pairs of a holetransporting layer adjacent to an electron transporting layer, the pairsdisposed between the cathode and the anode; and an exciton blockingdisposed between one of the cathode and the anode, and the plurality ofpairs.
 16. The device of claim 14 wherein one exciton blocking layer isdisposed between the anode and the plurality of pairs.
 17. The device ofclaim 14 wherein one exciton blocking layer is disposed between thecathode and the plurality of pairs.
 18. The device of claim 14 whereinthe exciton blocking layer is BCP
 19. The device of claim 14 wherein theexciton blocking layer is selected from the group consisting of m-MTDATAor PEDOT.
 20. The device of claim 12 wherein the hole transportinglayers and the electron transporting layers of the plurality of pairsare selected to have substantial spectral sensitivity in the visiblespectrum.
 21. The device of claim 12 wherein each pair of the pluralityof pairs are CuPc and PTCBI.
 22. The device of claim 12 wherein theplurality of pairs is at least 5 pairs.
 23. The device of claim 12wherein the plurality of pairs is at least 10 pairs.
 24. The device ofclaim 12 wherein an exciton blocking layer is disposed between the anodeand the plurality of pairs and the cathode and the plurality of pairs.